The invention concerns an endovascular implant comprising a tubular main body which is open at the ends and which is dilatable from an unexpanded condition into an expanded condition.
The implantation of endovascular support systems has been established for many years in medical technology as one of the most effective therapeutic measures in the treatment of vascular diseases. Thus for example, the introduction of stents in interventional therapy in respect of stable and unstable angina pectoris has resulted in a marked reduction in the rate of re-stenoses and thus better long-term results. The cause of using stent implantation in the above-indicated indication is the higher level of primary lumen gain. Admittedly, the use of stents makes it possible to achieve an optimum vessel cross-section which is primarily necessary for therapy success, but it will be noted that the permanent presence of a foreign body of that kind injects a cascade of microbiological processes which can result in the stent gradually growing shut. A starting point for resolving those problems is therefore that of making the stent from a biodegradable material.
A very wide range of different materials are available to the medical technologist, for implementing biodegradable implants of that kind. Besides numerous polymers which, for better biocompatibility, are frequently of natural origin or are at least based on natural compounds, in very recent times metallic materials are favored, with their mechanical properties which are substantially more advantageous in terms of the implants. In that connection, magnesium-bearing, iron-bearing and tungsten-bearing materials are particularly considered.
The endovascular implants are usually dilated with suitable catheter systems at the location of implantation. In general terms in that respect, a distinction can be drawn between self-expanding implants and implants which are only dilated by inflation of a balloon of the catheter system. Self-expanding implants can be formed for example, from shape memory materials such as Nitinol or they have structure elements, the interaction of which, leads mechanically to the desired expansion of the implant. In the case of balloon dilation, the implants which are crimped onto the balloon of the catheter system are uniformly deformed or expanded by the balloon.
One problem in the dilation of endovascular implants is that, when contact is made with the vessel wall or upon expansion of the vessel, the vessel wall suffers from very minor damage, tears and dissections which admittedly generally heal without problem but which can lead to proliferation phenomena due to the cell growth which is triggered off. That adverse effect is tolerated however, in order to prevent excessive contraction of the blood vessel after dilation, with a concomitant constriction in the vessel cross-section (obstruction). In order to counteract that effect, previous solutions provide, for example, applying coating systems to the implant, which contain pharmaceutical substances or which themselves deploy an effect of that kind. In that respect, the previous systems are essentially designed for the requirements involved in the treatment of greatly constricted or closed blood vessels with atherosclerotic plaque. Pathological tissue changes of that kind require a high level of radial strength for the implant in order to obviate obstruction of the vessel. It is admittedly known that vessel injuries result in an inflammatory response on the part of the tissue and ultimately promote neointimal hyperplasia (R. Kornowski, M. K. Hong, F. O. Tio, O. Bramwell, H. Wu, M. B. Leon, JACC, Vol. 31 (1998), No. 1, 224-230), but hitherto, the disadvantages outlined were tolerated in the absence of alternatives.
U.S. Pat. No. 5,888,201 provides that stents with a moderate degree of stiffness and a relatively low modulus of elasticity would be accessible to a wide range of uses. Described therein is a self-expanding stent structure comprising a biocompatible but non-biodegradable titanium alloy.
Besides the mechanical influences on the vessel wall upon dilation, the permanent presence of the implants of conventional nature also represents a re-stenosis point of attack. Frequently however, permanent presence of the implant is not necessary, from the medical point of view. There are therefore, endeavors to form the implant from a biodegradable material. Biodegradable metal alloys are know in this connection, for example based on magnesium, iron, zinc or tungsten.